Low-strain laser structures with group III nitride active layers

ABSTRACT

A Group III nitride laser structure is disclosed with an active layer that includes at least one layer of a Group III nitride or an alloy of silicon carbide with a Group III nitride, a silicon carbide substrate, and a buffer layer between the active layer and the silicon carbide substrate. The buffer layer is selected from the group consisting of gallium nitride, aluminum nitride, indium nitride, ternary Group III nitrides having the formula A x  B 1-x  N, where A and B are Group III elements and where x is zero, one, or a fraction between zero and one, and alloys of silicon carbide with such ternary Group III nitrides. In preferred embodiments, the laser structure includes a strain-minimizing contact layer above the active layer that has a lattice constant substantially the same as the buffer layer.

BACKGROUND OF THE INVENTION

The invention relates to semiconductor laser structures, and inparticular relates to laser structures formed from Group III nitrides(i.e., Group III of the Periodic Table of the Elements) that willproduce output in the blue to ultraviolet portions of theelectromagnetic spectrum.

FIELD OF THE INVENTION

A laser is a device that produces a beam of coherent or monochromaticlight as a result of stimulated emission. Light beams produced by laserscan have extremely high energy because of their single wavelength,frequency, and coherence. A number of materials are capable of producingthe lasing effect and include certain high-purity crystals (ruby is acommon example), semiconductors, certain types of glass, certain gasesincluding carbon dioxide, helium, argon and neon, and certain plasmas.

High power lasers are useful in industry for cutting difficult materialssuch as metals, composites and diamonds. Lasers are also used to promotechemical reactions (e.g., flash photolysis) as well as for spectroscopyand photography. Laser techniques are also applied in medicine andsurgery. High-powered lasers are used for controlled fusion reactionswhile others are used for biomedical investigations, for organicchemical research, for other sophisticated analytical techniques, and inthree-dimensional photography (holography).

More recently, lasers have been developed in semiconducting materials,thus taking advantage of the smaller size, lower cost and other relatedadvantages typically associated with semiconductor devices. In thesemiconductor arts, devices in which photons play a major role arereferred to as "photonic" or "optoelectronic" devices. In turn, photonicdevices include light-emitting diodes (LEDs), photodetectors,photovoltaic devices, and semiconductor lasers.

Semiconductor lasers are similar to other lasers in that the emittedradiation has spacial and temporal coherence. As noted above, laserradiation is highly monochromatic (i.e., of narrow band width) and itproduces highly directional beams of light. Semiconductor lasers differ,however, from other lasers in several respects. For example, insemiconductor lasers, the quantum transitions are associated with theband properties of materials; semiconductor lasers are very compact insize, have very narrow active regions, and larger divergence of thelaser beam; the characteristics of a semiconductor laser are stronglyinfluenced by the properties of the junction medium; and for p-njunction lasers, the lasing action is produced by passing a forwardcurrent through the diode itself. Overall, semiconductor lasers providevery efficient systems that are easily modulated by modulating thecurrent directed across the devices. Additionally, because semiconductorlasers have very short photon lifetimes, they could be used to producehigh-frequency modulation. In turn, the compact size and capability forsuch high-frequency modulation make semiconductor lasers an importantlight source for optical fiber communications.

In broad terms, the structure of a semiconductor laser should meet threebroad requirements: optical confinement, electrical confinement, andmirroring. Additionally, in order to produce the laser effect(stimulated emission of radiation), the semiconductor must be a directbandgap material rather than an indirect bandgap material. As known tothose familiar with semiconductor characteristics, a direct bandgapmaterial is one in which an electron's transition from the valence bandto the conduction band does not require a change in crystal momentum forthe electron. Gallium arsenide and gallium nitride are examples ofdirect semiconductors. In indirect semiconductors, the alternativesituation exists; i.e., a change of crystal momentum is required for anelectron's transition between the valence and conduction bands. Siliconand silicon carbide are examples of such indirect semiconductors.

A useful explanation of the theory, structure and operation ofsemiconductor lasers, including optical and electronic confinement andmirroring, is given by Sze, Physics of Semiconductor Devices, 2ndEdition (1981) at pages 704-742, and these pages are incorporatedentirely herein by reference.

As known to those familiar with photonic devices such as LEDs andlasers, the frequency of electromagnetic radiation (i.e., the photons)that can be produced by a given semiconductor material are a function ofthe material's bandgap. Smaller bandgaps produce lower energy, longerwavelength photons, while wider bandgap materials are required toproduce higher energy, shorter wavelength photons. For example, onesemiconductor commonly used for lasers is indium gallium aluminumphosphide (InGaAlP). Because of this material's bandgap (actually arange of bandgaps depending upon the mole or atomic fraction of eachelement present), the light that InGaAlP can produce is limited to thered portion of the visible spectrum, i.e., about 600 to 700 nanometers(nm).

Working backwards, in order to produce photons that have wavelengths inthe blue or ultraviolet portions of the spectrum, semiconductormaterials are required that have relatively large bandgaps. Typicalcandidate materials include silicon carbide (SiC) and gallium nitride(GaN).

Shorter wavelength lasers offer a number of advantages in addition tocolor. In particular, when used in optical storage and memory devices(e.g., "CD-ROM" or "optical disks"), their shorter wavelengths enablesuch storage devices to hold proportionally more information. Forexample, an optical device storing information using blue light can holdapproximately 32 times as much information as one using red light, inthe same space.

Silicon carbide has been demonstrated to produce excellentlight-emitting diodes (LEDs) in the blue and ultraviolet (UV) range ofthe spectrum (and correspondingly photodetectors as well), but is anindirect bandgap material, and thus although useful for LEDs, will notproduce the laser effect.

Gallium nitride, however, is an attractive laser candidate material forblue and UV frequencies because of its relatively high bandgap (3.36 eVat room temperature). Gallium nitride suffers from a differentdisadvantage, however: the failure to date of any workable technique forproducing bulk single crystals of gallium nitride which could formappropriate substrates for gallium nitride photonic devices. As is knownto those familiar with semiconductor devices, they all require some sortof structural substrate, and typically a substrate formed of the samematerials as the active region of a device offers significantadvantages, particularly in crystal growth and matching. Because galliumnitride has yet to be formed in such bulk crystals, however, galliumnitride photonic devices must be formed in epitaxial layers on differentsubstrates.

Using different substrates, however, causes an additional set ofproblems, mostly in the area of crystal lattice matching. In almost allcases, different materials have different crystal lattice parameters. Asa result, when a gallium nitride epitaxial layer is grown on a differentsubstrate, some crystal mismatch will occur, and the resulting epitaxiallayer is referred to as being "strained" by this mismatch. Suchmismatches, and the strain they produce, carry with them the potentialfor crystal defects which in turn affect the electronic characteristicsof the crystals and the junctions, and thus correspondingly tend todegrade or even prevent the performance of the photonic device. Suchdefects are even more problematic in laser structures becausesemiconductor lasers typically operate at electrical power levels atleast an order of magnitude greater than those at which LEDs operate.

To date, the most common substrate for gallium nitride devices--and theonly substrate utilized in GaN LED's--has been sapphire; i.e., aluminumoxide (Al₂ O₃). Sapphire is optically transparent in the visible and UVranges, but is unfortunately insulating rather than conductive, andcarries a lattice mismatch with gallium nitride of about 16%. In theabsence of a conductive substrate, "vertical" devices (those withcontacts on opposite sides) cannot be formed, thus complicating themanufacture and use of the devices.

As a particular disadvantage, horizontal structures (those with contactson the same side of the device), such as those required when galliumnitride is formed on sapphire, also produce a horizontal flow of currentdensity. This horizontal current flow puts an additional strain on thealready-strained (i.e., the 16% lattice mismatch) GaN-sapphirestructure.

Gallium nitride also carries a lattice mismatch of about 2.4% withaluminum nitride (AlN) and a 3.5% mismatch with silicon carbide. SiliconCarbide has a somewhat lesser mismatch (only about 1%) with aluminumnitride.

Group III ternary and quaternary nitrides (e.g., InGaN, InGaAlN, etc.)have also been shown to have relatively wide bandgaps and thus alsooffer the potential for blue and ultraviolet semiconductor lasers. Mostof these compounds, however, present the same difficulty as galliumnitride: the lack of an identical single crystal substrate. Thus, eachmust be used in the form of epitaxial layers grown on differentsubstrates. Thus, they present the same potential for crystals defectsand their associated electronic problems.

OBJECT AND SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide Group IIInitride laser structures that meet all of the requirements for lasercapability and that can be formed on conductive substrates of siliconcarbide.

The invention meets this object with a separate-confinementheterostructure laser that includes an indium gallium nitride activelayer, upper and lower waveguide layers on the active layer and formedof aluminum gallium nitride or indium gallium nitride, upper and lowercladding layers on the respective upper and lower waveguide layersformed of aluminum gallium nitride, and a strain-minimizing aluminumgallium nitride contact layer on the upper cladding layer.

In another aspect, the invention comprises a substrate structure thatwill support such a Group III nitride laser structure with minimumstrain. In this aspect, the invention comprises a silicon carbidesubstrate, a first buffer layer upon the substrate formed of a gradedcomposition of silicon carbide aluminum gallium nitride ((SiC)_(x)(Al_(y) Ga_(1-y) N)_(1-x) ; where 0≦x, y≦1)), and a second buffer layerupon the first layer and formed of a graded composition of aluminum andgallium nitride (Al_(y) Ga_(1-y) N).

In yet another embodiment, the invention comprises a Group III nitridelaser structure that has an active layer that includes at least onelayer of a Group III nitride, a silicon carbide substrate, and acoupling structure between the active layer and the substrate.

In a further aspect the invention comprises a Group III nitride laserstructure with an active layer that includes at least one layer of aGroup III nitride, a silicon carbide substrate, and a buffer layerbetween the active layer and the silicon carbide substrate. The bufferlayer is selected from the group consisting of gallium nitride, aluminumnitride, indium nitride, and ternary Group III nitrides having theformula A_(x) B_(1-x) N, where A and B are Group III elements and wherex is zero, one, or a fraction between zero and one. The structure canfurther include a strain-minimizing contact layer above the active layerthat has a lattice constant substantially the same as the buffer layer.

The foregoing and other objects, advantages and features of theinvention, and the manner in which the same are accomplished, willbecome more readily apparent upon consideration of the followingdetailed description of the invention taken in conjunction with theaccompanying drawings, which illustrate preferred and exemplaryembodiments, and wherein:

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional representation of a separate confinementheterostructure laser according to the present invention;

FIG. 2 is a cross-sectional representation of a substrate structure thatwill support a Group III nitride laser structure according to thepresent invention;

FIG. 3 is a cross-sectional representation of a Group III nitride laserstructure according to the present invention;

FIG. 4 is a plot of relative intensity versus hours of operation (i.e.,degradation curve) for GaN LEDs on sapphire and the expected degradationfor GaN LEDs on silicon carbide according to the present invention;

FIG. 5 is an auger electron spectrum (AES) for a typical (SiC)_(x)(Al_(y) Ga_(1-y) N)_(1-x) layer according to the present invention;

FIG. 6 is a plot of photoluminescence (luminescence versus energy in eV)for semiconductor LEDs according to the present invention;

FIG. 7 is a double crystal x-ray rocking curve (DCXRC) for GaN on SiCaccording to the present invention; and

FIG. 8 is a plot of the cathodoluminescence (CL) edge peak energy as afunction of SiC concentration in a SiC-AlGaN alloy according to thepresent invention.

DETAILED DESCRIPTION

The present invention is a laser diode structure which is grown on alattice mismatched silicon carbide substrate. As used herein, the term"lattice mismatched" means that one or more layers of the laserstructure have crystal lattice parameters different from the crystallattice parameters of silicon carbide. The structure of the inventionbalances the individual strains of each layer to produce a structurewith minimal net strain. FIG. 1 illustrates one embodiment of aseparate-confinement heterostructure (SCH) indium gallium nitride laserbroadly designated at 10 according to the present invention. The laserstructure 10 includes an indium gallium nitride active layer 11. Upperand lower waveguide layers 12 and 13 respectively bracket the activelayer and provide part of the waveguiding required for a laser of thistype. The waveguide layers 12 and 13 are preferably formed of aluminumgallium nitride. Upper and lower cladding layers 14 and 15 respectivelyare on the respective upper and lower waveguide layers 12 and 13 and arealso formed of aluminum gallium nitride, thus completing the opticalwaveguide structure.

In preferred embodiments, a strain minimizing layer 16 which is formedof aluminum gallium nitride and also serves as a top contact layer is onthe upper cladding layer 14. The structure also includes a buffer layer17 and an appropriate substrate 20, preferably formed of siliconcarbide. Optionally a very thin layer of gallium nitride (not shown) canbe added to the strain minimizing layer 16 and can serve as the topcontact layer for the overall structure.

As known to those familiar with semiconductor laser structures, in orderto enhance the laser capabilities of the device, particularly a separateconfinement heterostructure such as the present invention, the activelayer should desirably have a lower bandgap than the adjacent waveguideand cladding layers, and a higher refractive index than the adjacentwaveguide and cladding layers. Such a structure gives two benefitsimportant for laser capability. First, if the active layer has thelowest bandgap, it forms a quantum well into which electrons tend tofall, thus helping enhance the laser effect. Second, waveguiding occursin the material that has the highest refractive index in the structure.Accordingly, when the bandgap of the active layer is less than that ofthe adjacent layers, and its refractive index is greater than that ofthe adjacent layers, the lasing capabilities of the device are enhanced.

Accordingly, in FIG. 1, the indium gallium nitride layer 11 preferablyhas the composition In_(z) Ga_(1-z) N, where z is a fraction between 0and 1. As known to those familiar with Group III elements (e.g., Sze p.706), when compound semiconductors are formed that have more than oneGroup III element, these compounds are present as crystalline solidsolutions. The formulations are often given in alphabetical order (Al,Ga, In), or in order of atomic number (aluminum 13, gallium 31, indium49). In broad terms, the Group III nitride compounds described hereinfor the active, waveguide, cladding, and buffer layers can include thefollowing: gallium nitride; aluminum nitride; indium nitride; ternaryGroup III nitrides having the formula A_(x) B_(1-x) N, where A and B areGroup III elements and where x is zero, one, or a fraction between zeroand one; quaternary Group III nitrides having the formula A_(x) B_(y)C_(1-x-y) N, where A, B and C are Group III elements and where x and yare zero, one, or a fraction between zero and one and 1 is greater than(x+y); and alloys of silicon carbide with such ternary and quaternaryGroup III nitrides.

In such Group III compounds, the bandgap of the compound semiconductortends to increase as the proportion of aluminum increases and the indexof refraction increases as the proportion of aluminum decreases.Accordingly, in a preferred embodiment, the waveguide layers 12 and 13have the formula Al_(x) Ga_(1-x) N, where x is 0, 1, or a fractionbetween 0 and 1. As Table IIb indicates, however, in some embodimentsthe waveguide layers can contain indium as well. Because the waveguidelayers have more aluminum (i.e., a greater atomic or mole fraction) thanthe active layer, they have a larger bandgap, thus encouraging electronsto move into the active layer in the desired manner. Expresseddifferently, the waveguide layers 12 and 13 have bandgaps larger thanthe active layer 11.

For the reasons just stated with respect to refractive index, thewaveguide layers preferably have refractive indexes that are smallerthan the active layer so that they encourage wave propagation to occurin the active layer as is most desirable for a laser structure.

In turn, the cladding layers 14 and 15 have the formula Al_(y) Ga_(1-y)N wherein y is greater than x and where y is 0, 1, or a fraction between0 and 1. In other words, the cladding layers have a greater atomicfraction of aluminum in the compound semiconductor than do the waveguidelayers 12 and 13, and thus have a higher bandgap and a smallerrefractive index than the waveguide layers.

Finally, the strain minimizing contact layer 16 has the formula Al_(w)Ga_(1-w) N wherein w is greater than x and less than y and where w is 1,0, or a fraction between 1 and 0.

In preferred embodiments, the laser structure of FIG. 1 is constructedso that the upper waveguide and upper cladding layers 12 and 14 have thesame conductivity type as each other, and the lower waveguide and lowercladding layers 13 and 15 have the same conductivity type and that isopposite to the conductivity type of the upper layers. This permits, ofcourse, current injection through the device between the oppositeconductivity type portions. In the most preferred structures, the upperlayers 12 and 14 are p-type, the lower layers 13 and 15 are n-type, andthe strain minimizing top contact layer 16 is p-type.

In addition to the optical and electronic characteristics of thepreferred structure, the embodiment illustrated in FIG. 1 also minimizesthe strain between layers of different materials that have differentcrystal lattices. In the invention, each of the waveguide and claddinglayers has a thickness sufficient to accommodate the strain between itand its adjacent layers, but less than the thickness at which misfitdislocations begin to form. In this regard, it will be understood bythose familiar with semiconductor crystal structures that strain effectsare present any time two different materials are adjacent to oneanother. As a result, the preferred thickness for an epitaxial layer isa thickness that is appropriate for the other performance parameters ofthe device, but less than the critical thickness. The critical thicknessis the maximum thickness that the layer can be grown in strained fashionbefore dislocations begin to propagate. The strain ("ε") between twolayers is often expressed as the difference in the crystal latticeparameters between the two layers (Δa) divided by the lattice parameterof one of the layers (a), often the substrate. The higher this strainvalue, the thinner the layer that can be grown between the twomaterials. Furthermore, in a multilayer structure such as illustrated inFIG. 1, the overall strain ("Σ") is a function or summation of theindividual layer strains, and is referred to as the "effective strain."

Accordingly, a preferred embodiment of the laser structure of FIG. 1includes the silicon carbide substrate 20 and the buffer layer 17 whichis-formed of one or more layers having a composition selected from thegroup consisting of gallium nitride, aluminum nitride, and Al_(x)Ga_(1-x) N (0≦×≦1). Such a buffer is described in copending applicationSer. No. 08/166,229 filed Dec. 13, 1993, which is commonly assigned withthe pending application, and which is incorporated entirely herein byreference.

Additionally, in a preferred embodiment, the strain minimizing contactlayer 16 has a lattice constant that matches as closely as possible thelattice constant of the buffer layer 17. Although these two layers arenot in contact with one another, matching their lattice constantsnevertheless reduces strain. Strain is minimized when the latticeconstant of the buffer layer is substantially the same as the average ofthe layers above the buffer layer. By adding the strain-reducing layer16 with essentially the same lattice constant as the buffer layer 17,the average lattice constant of the layers above the buffer moves moreclosely toward the buffer and thus reduces the overall strain of theentire structure. Stated differently, the presence of the top layer 16moves the weighted average of all of the layers 11, 12, 13, 14, 15 and16 closer to the lattice constant of the buffer layer 17 and thusreduces the overall strain.

Accordingly, in preferred embodiments, the lattice constant of thebuffer layer 17 is substantially the average of the lattice constants ofthe active layer 11, the waveguide layers 12 and 13, the cladding layers14 and 15, and the strain minimizing contact layer 16.

Exemplary compositions of a laser structure similar to that illustratedat 10 in FIG. 1 are set forth in the following tables in which λrepresents the laser's emission wavelength and d/Γ represents the ratioof the active layer thickness to the confinement factor. In laserdiodes, d/Γ is proportional to the threshold current density, and a lowvalue is desired. The layer thicknesses are expressed in angstroms(10⁻¹⁰ meter).

                  TABLE I                                                         ______________________________________                                        Strain-optimized strained-layer laser structure for                           10% In active region (λ.sub.g = nm, d/Γ = 0.221).                Layer           Composition                                                                              Thickness                                          ______________________________________                                        Top/Contact Layer                                                                             Al.sub.0.10 Ga.sub.0.90 N                                                                10000A                                             Upper Cladding  Al.sub.0.12 Ga.sub.0.88 N                                                                4000                                               Upper Waveguide Al.sub.0.01 Ga.sub.0.99 N                                                                700                                                Active Region   In.sub.0.10 Ga.sub.0.90 N                                                                100                                                Lower Waveguide Al.sub.0.01 Ga.sub.0.99 N                                                                700                                                Lower Cladding  Al.sub.0.12 Ga.sub.0.88 N                                                                4000                                               Buffer Layer    Al.sub.0.10 Ga.sub.0.90 N                                                                --                                                 ______________________________________                                    

                  TABLE IIa                                                       ______________________________________                                        Strain-optimized strained-layer laser structure for                           15% In active region (λ.sub.g = 415 nm, d/Γ = 0.286).            Layer           Composition                                                                              Thickness                                          ______________________________________                                        Top/Contact Layer                                                                             Al.sub.0.15 Ga.sub.0.85 N                                                                10000A                                             Upper Cladding  Al.sub.0.17 Ga.sub.0.83 N                                                                4000                                               Upper Waveguide Al.sub.0.04 Ga.sub.0.96 N                                                                500                                                Active Region   In.sub.0.15 Ga.sub.0.85 N                                                                 60                                                Lower Waveguide Al.sub.0.04 Ga.sub.0.96 N                                                                500                                                Lower Cladding  Al.sub.0.17 Ga.sub.0.83 N                                                                4000                                               Buffer Layer    Al.sub.0.15 Ga.sub.0.85 N                                                                --                                                 ______________________________________                                    

                  TABLE IIb                                                       ______________________________________                                        Strain-optimized strained-layer laser structure for                           15% In active region (λ.sub.g = 415 nm, d/Γ = 0.303).            Layer           Composition                                                                              Thickness                                          ______________________________________                                        Top/Contact Layer                                                                             GaN         5000A                                             Upper Cladding  Al.sub.0.02 Ga.sub.0.98 N                                                                4000                                               Upper Waveguide In.sub.0.02 Ga.sub.0.98 N                                                                1000                                               Active Region   In.sub.0.15 Ga.sub.0.85 N                                                                 90                                                Lower Waveguide In.sub.0.02 Ga.sub.0.98 N                                                                1000                                               Lower Cladding  Al.sub.0.02 Ga.sub.0.98 N                                                                4000                                               Buffer Layer    GaN        --                                                 ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Strain-optimized strained-layer laser structure for                           20% In active region (λ.sub.g = 430 nm, d/Γ = 0.322).            Layer           Composition                                                                              Thickness                                          ______________________________________                                        Top/Contact Layer                                                                             Al.sub.0.10 Ga.sub.0.90 N                                                                 5000A                                             Upper Cladding  Al.sub.0.12 Ga.sub.0.88 N                                                                4000                                               Upper Waveguide Al.sub.0.01 Ga.sub.0.99 N                                                                600                                                Active Region   In.sub.0.20 Ga.sub.0.80 N                                                                 50                                                Lower Waveguide Al.sub.0.01 Ga.sub.0.99 N                                                                600                                                Lower Cladding  Al.sub.0.12 Ga.sub.0.88 N                                                                4000                                               Buffer Layer    Al.sub.0.10 Ga.sub.0.90 N                                                                --                                                 ______________________________________                                    

For all the reasons just discussed, and in addition to producing aminimized strained laser structure, it will be clearly understood thatthe substrate structure--i.e. the bulk single crystal substrate and anybuffer layers upon it--likewise should be optimized to obtain successfullaser structures of the type described herein. Thus, in a second aspectof the invention, and as illustrated at 22 in FIG. 2, an appropriatesubstrate structure includes a silicon carbide substrate 23 and firstand second buffer layers 24 and 25 upon the substrate. The first bufferlayer 24 is formed of a graded composition of silicon carbide andaluminum gallium nitride ((SiC)_(x) (Al_(y) Ga_(1-y) N)_(1-x)) in whichthe portion adjacent the substrate is substantially entirely siliconcarbide and the portion furthest from the substrate is substantiallyentirely aluminum gallium nitride (Al_(y) Ga_(1-y) N) with the portionstherebetween being progressively graded in content from predominantlysilicon carbide to predominantly Al_(y) Ga_(1-y) N. The grading is interms of atomic fraction and is possible because of the good alloyingproperties of silicon carbide with Group III nitrides. Using appropriateepitaxial growth techniques, the grading can be controlled to producethe layers described.

The second buffer layer 25 on the first layer 24 is formed of asimilarly graded composition, but one of graded aluminum galliumnitride, i.e., from Al_(y) Ga_(1-y) N to Al_(z) Ga_(1-x) N, where "z"can differ from "y." In the second buffer layer 25, the portion adjacentthe first layer 24 is a first composition of Al_(y) Ga_(1-y) N and theportion furthest from the first layer is a second composition of Al_(z)Ga_(1-z) N with the portions therebetween being progressively graded incontent from one aluminum gallium nitride composition to the other. Asset forth earlier, these various compositions of Group III nitridesexist in crystalline form as solid solutions and thus the second bufferlayer 25 can be appropriately and uniformly graded in the same manner asbuffer layer 24.

In FIG. 2, the second buffer layer 25 of aluminum gallium nitride canhave one of the desired compositions set forth in Table 1. In a mostpreferred embodiment, the Al_(z) Ga_(1-z) N composition portion of thegraded second layer 25 matches the aluminum gallium nitride compositionof the lower cladding layer of a laser structure built upon thesubstrate structure 22. Such a composition can, of course, includecompositions matching those set forth in Tables I, IIa, IIb, and III forthe lower cladding layer 15.

FIG. 3 shows a third embodiment broadly designated at 30 in which anentire laser structure is schematically illustrated. The structureincludes an active layer 31 that has at least one layer of a Group IIInitride, a silicon carbide substrate 32, and a coupling structure 33between the active layer 31 and the substrate 32, the coupling layerincluding at least one graded layer of silicon carbide and a Group IIInitride in which the graded layer is silicon carbide at the interfacewith the substrate 32, and the graded layer is the Group III nitride atthe interface with the active layer 31.

The remaining figures demonstrate further advantages and beneficialcharacteristics of the present invention.

FIG. 4 is a plotted curve showing the known degradation of galliumnitride LEDs on sapphire and the expected degradation of gallium nitrideLEDs on silicon carbide, with the prediction being based upon the knowncharacteristics of silicon carbide and silicon carbide devices, andtheir demonstrated performances to date.

FIG. 5 is an auger electron spectrum of a silicon carbide aluminumgallium nitride layer according to the present invention demonstratingthat all five elements (Si, C, Al, Ga, N) were present in the layerexamined. High energy electron diffraction of layers having this augerelectron spectrum showed that they were single crystal layers.

FIG. 6 is a plot of photoluminescence versus energy for gallium nitrideon silicon carbide and demonstrating a sharp emission peak maximized at3.41 eV, with a full width at half maximum of only 38.6 meV.

FIG. 7 is a double-crystal x-ray rocking curve (DCXRC) for galliumnitride on silicon carbide and showing a full width at half maximum ofonly 85 arc seconds. As is known to those familiar with the evaluationof crystals and the DCXRC technique, the relatively small value for thefull width at half maximum demonstrates excellent crystal properties.

Finally, FIG. 8 is a plot of the CL edge peak energy as a function ofthe mole percentage of silicon carbide (the SiC concentration) in(SiC)_(x) (Al_(y) Ga_(1-y) N)_(1-x) alloys according to the presentinvention. The CL measurements performed on this layer at 80K showedseveral peaks in the ultraviolet and violet regions. The dominant UVpeak belonged to the GaN layer, but additional UV peaks with photonenergies higher than the GaN peak were also observed. The photon energyfor the dominant peak depends on the SiC concentration in the alloylayer as demonstrated by FIG. 8. For a layer with a SiC concentration ofapproximately 20 mole percent, the peak was detected at a wavelength ofabout 300 nanometers.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms havebeen employed, they have been used in a generic and descriptive senseonly and not for purposes of limitation, the scope of the inventionbeing set forth in the following claims.

That which is claimed is:
 1. A separate-confinement heterostructurelaser comprising:an indium gallium nitride active layer; upper and lowerwaveguide layers on said active layer formed from the group consistingof AlGaN, InGaN, and AlInGaN; and upper and lower cladding layers onsaid respective upper and lower waveguide layers and formed of AlGaN. 2.A laser according to claim 1 and further comprising a strain minimizingAlGaN contact layer on said upper cladding layer.
 3. A laser accordingto claim 1 wherein said waveguide layers have the formula Al_(x)Ga_(1-x) N, and where x is 0, 1, or a fraction between 0 and
 1. 4. Alaser according to claim 1 wherein said waveguide layers have theformula Al_(x) Ga_(y) IN_(1-x-y) N, and where x and y are 0, 1, or afraction between 0 and 1 and where 1 is greater than (x+y).
 5. A laseraccording to claim 1 wherein said waveguide layers have a bandgap largerthan said active layer.
 6. A laser according to claim 1 wherein saidwaveguide layers have a refractive index smaller than said active layer.7. A laser according to claim 3 wherein said cladding layers have theformula Al_(y) Ga_(1-y) N wherein y is greater than x, and where y is 0,1, or a fraction between 0 and
 1. 8. A laser according to claim 1wherein said cladding layers have a bandgap larger than said waveguidelayers.
 9. A laser according to claim 1 wherein said cladding layershave a refractive index smaller than said waveguide layers.
 10. A laseraccording to claim 7 and further comprising a strain minimizing AlGaNcontact layer on said upper cladding layer, and wherein said strainminimizing contact layer has the formula Al_(w) Ga_(1-w) N wherein w isgreater than x and less than y, and where w is 0, 1, or a fractionbetween 0 and
 1. 11. A laser according to claim 1 wherein:said upperwaveguide and upper cladding layers have the same conductivity type;said lower waveguide and lower cladding layers have the sameconductivity type and that is opposite to the conductivity type of saidupper layers.
 12. A laser according to claim 11 wherein said upperlayers are p-type and said lower layers are n-type.
 13. A laseraccording to claim 12 and further comprising a strain minimizing AlGaNcontact layer on said upper cladding layer, and wherein said strainminimizing contact layer is p-type.
 14. A laser according to claim 1wherein each of said waveguide and cladding layers has a thicknesssufficient to substantially accommodate the strain between it and itsadjacent layers, but less than the thickness at which misfitdislocations begin to form.
 15. A laser according to claim 1 and furthercomprising:a silicon carbide substrate; and a buffer layer formed of oneor more layers having a composition selected from the group consistingof GaN, AlN, InN, and Al_(x) Ga_(1-x) N, and where x is zero, one, or afraction between zero and one.
 16. A laser according to claim 15 andfurther comprising a strain minimizing AlGaN contact layer on said uppercladding layer, and wherein said strain-minimizing contact layer has alattice constant that matches said buffer layer.
 17. A laser accordingto claim 15 and further comprising a strain minimizing AlGaN contactlayer on said upper cladding layer, and wherein the lattice constant ofsaid buffer layer is substantially the average of the lattice constantsof said active layer, said waveguide layers, said cladding layers andsaid strain-minimizing contact layer.
 18. A laser structure according toclaim 1 wherein said active, waveguide, cladding, and contact layershave the composition and thicknesses set forth in Table I.
 19. A laserstructure according to claim 1 wherein said active, waveguide, cladding,and contact layers have the composition and thicknesses set forth inTable IIa.
 20. A laser structure according to claim 1 wherein saidactive, waveguide, cladding, and contact layers have the composition andthicknesses set forth in Table IIb.
 21. A laser structure according toclaim 1 wherein said active, waveguide, cladding, and contact layershave the composition and thicknesses set forth in Table III.
 22. Aseparate-confinement heterostructure laser comprising:an aluminum indiumgallium nitride active layer; upper and lower waveguide layers on saidactive layer formed from the group consisting of AlGaN, InGaN, andAlInGaN; and upper and lower cladding layers on said respective upperand lower waveguide layers and formed of AlGaN.
 23. A laser structureaccording to claim 22 wherein said active layer has the compositionAl_(x) Ga_(y) In_(1-x-y) N, and where x and y are 0, 1, or a fractionbetween 0 and 1 and where 1 is greater than (x+y).
 24. A group IIInitride laser structure comprising:an active layer that includes atleast one layer of a Group III nitride; a silicon carbide substrate; anda coupling structure between said active layer and said substrate, saidcoupling layer including at least one graded layer of silicon carbideand a group III nitride in which the graded layer is silicon carbide atthe interface with said substrate, and the graded layer is the group IIInitride at the interface with said active layer.
 25. A laser accordingto claim 24 wherein said Group III nitride in said active layer isselected from the group consisting of gallium nitride, aluminum nitride,indium nitride, ternary Group III nitrides having the formula A_(x)B_(y) C_(1-x-y) N, where A, B and C are Group III elements and where xand y are zero, one, or a fraction between zero and one and 1 is greaterthan (x+y), and alloys of silicon carbide with such ternary Group IIInitrides.
 26. A laser according to claim 25 and further comprising astrain-minimizing layer above said active layer, said strain-minimizinglayer having a lattice constant that is substantially the same as thelattice constant of said coupling structure.
 27. A laser according toclaim 24 wherein said active layer further comprises:respective upperand lower waveguide layers adjacent both surfaces of said active layer;an upper cladding layer on said upper waveguide layer; and a lowercladding layer below said lower waveguide layer.
 28. A laser accordingto claim 27 wherein said waveguide layers have a wider bandgap and alower refractive index than said active layer; andsaid cladding layershave a wider bandgap and a lower refractive index than said waveguidelayers.
 29. A laser according to claim 27 wherein said strain minimizinglayer is on said upper cladding layer.
 30. A laser according to claim 29wherein said lower cladding layer is on said coupling structure.
 31. Agroup III nitride laser structure comprising:an active layer thatincludes at least one layer of a Group III nitride; a silicon carbidesubstrate; a buffer layer between said active layer and said siliconcarbide substrate, said buffer layer being selected from the groupconsisting of gallium nitride, aluminum nitride, indium nitride, ternaryGroup III nitrides having the formula A_(x) B_(1-x) N, where A and B areGroup III elements and where x is zero, one, or a fraction between zeroand one, and alloys of silicon carbide with such ternary Group IIInitrides; and a strain-minimizing contact layer above said active layerthat has a lattice constant substantially the same as said buffer layer.32. A laser structure according to claim 31 wherein said active layerfurther comprises:respective upper and lower waveguide layers adjacentboth surfaces of said active layer; an upper cladding layer on saidupper waveguide layer; and a lower cladding layer below said lowerwaveguide layer.
 33. A laser structure according to claim 32 whereinsaid strain minimizing layer is on said upper cladding layer.
 34. Alaser structure according to claim 32 wherein said lower cladding layeris on said buffer layer.
 35. A laser according to claim 32 wherein saidwaveguide layers have a wider bandgap and a lower refractive index thansaid active layer; andsaid cladding layers have a wider bandgap and alower refractive index than said waveguide layers.